A coarse wave-length division multiplexing (CWDM) transmit module typically has 4, 8 or 16 channels. The CWDM laser wavelengths, for example, may cover a range of 1310 nm to 1610 nm with a spacing of 1-25 nm. A common spacing may be 1310 nm, 1330 nm, 1350 nm . . . , and so forth for 16 divisions based on a 20 nm separation. The CWDM transmit module combines the various laser wavelengths for transmission along fiber optic cables.
A CWDM transmit module may utilize ring lasers disposed in a semiconductor substrate. The main wavelength range of emission of each of the ring laser channels is determined by both the cavity design (ring radius) and the III-V semiconductor material (e.g. Al, GA, In and N, P As, Sb) and its associated gain spectrum.
Due to possible fabrication variation of the underlying silicon, thermal fluctuations and other process/fabrication fluctuations may occur. It may not be possible to absolutely and deterministically predict the wavelength of emission of a single laser cavity. Furthermore, it may be difficult to accurately and deterministically predict the relative channel spacing of a multi-wavelength system.
The ring laser wavelengths may need to be tuned to achieve specific spacing by shifting the emitted ring laser wavelengths to maintain or achieve a desired wavelength spacing. Furthermore, once tuned, it is desirable to allow the wavelength of emission to remain static even though environmental temperatures will change and thus cause a shift in the channel wavelength of emission.